The 'do-so'-diagnostic has been employed for decades to motivate two central aspects of constituency-based theories of syntax: the binary subject–predicate division of the clause (S→NP VP) and the layered analysis of VPs in general. Scrutiny of the diagnostic reveals that it actually supports neither of these two central aspects of most constituency grammars. This manuscript demonstrates that the distribution of the pro-verb do so can be accurately characterized in terms of the catena. The catena is a unit of syntactic analysis associated with dependency grammar (DG). It is a more inclusive and versatile unit than the constituency grammar constituent and it is therefore better suited than the constituent to serve as the fundamental unit of syntactic analysis. In the end, the catena-based account of do so delivers support for the flat analysis of (non-finite) VPs.